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Intelligent Design and Common Descent

Reader Colin C. asks:

I was just wondering why some fellows at Discovery believe in ID but still hold to common descent. Science knows that the genetic code is not universal. Different organisms use different genetic codes. It seems this fact would disprove common descent. I’ve read both Michael Behe’s books and he never talks about this. Even Dawkins mentioned this in The Greatest Show on Earth. I would appreciate an answer. Thanks.

My reply:

Dear Colin,

Thanks for writing. Your question about common descent is a common one among our readers, so I will try to explain some of the issues in a short response here. What I won’t do is go into all the evidence for and against common descent.  That would require a book-length treatment.

I first need to make clear that living things can be the product both of intelligent design and of common descent. If the designer chose to guide the process of gradual change from species to species, that would be both common descent and  intelligent design. In other words, intelligent design theory does not require that common descent is false. Neither does intelligent design require that common descent is true. All that intelligent design theory says is that the best explanation for what we see in the universe, and most particularly in life, is intelligence — that intelligence had to be involved in producing the living things that we see around us. Neo-Darwinism or any other strictly materialist process cannot create the diversity, intricacy, and splendor that we observe. The mechanism of mutation and natural selection is not sufficient.

That’s the key to intelligent design theory. It’s not about whether or not life evolved from one or even multiple common ancestors. It’s about whether life required intelligent design in its origin or diversification.

You make the point that the genetic code is not universal, that there are differences in the code among different groups of organisms. This is a true statement — however, the differences are not huge. It’s not as if they were completely separate unique codes. It’s possible to use the codes’ similarity as an argument for common descent (if one discounts design automatically). At the same time, code differences argue against common descent, because any change to a code is going to be disruptive. But it does not disprove common descent and here’s why. There are two kinds of common ancestry — there is universal common ancestry, meaning all creatures come from a single common ancestor, and there is just plain old common ancestry, meaning that some lineage of creatures shares a common ancestor. In that sense all creatures have a common ancestor they share with other members of their species, or perhaps genus or family. It may be that other groups small or large may share common ancestry, but that does not mean that we all came from one last universal common ancestor, namely LUCA. 

Some ID proponents accept common ancestry, with the caveat that the process needed guidance. Some do not accept common ancestry either in the narrow sense or the universal sense. I would say many do not accept universal common ancestry, but I don’t think a poll has ever been taken. Let me reiterate, however, that it is not a critical issue for intelligent design. The truth of intelligent design does not rest on whether we are descended from one or several common ancestors.

We are not weaseling or waffling. It’s just that common ancestry is not ruled out automatically by intelligent design. The designer could have used common ancestry if he wanted. Now, I would add that in my opinion, there is solid evidence that the designer probably used guided common descent and intelligent design. However, I don’t have time to write about all the science surrounding the issue of common descent (or common ancestry). So I will have to leave it there. There are several chapters in the book Theistic Evolution: A Scientific, Philosophical, and Theological Critique that discuss this topic, and Jonathan Wells also does in his book Zombie Science.

Image: Darwin’s Tree (1837), a sketch from his First Notebook on Transmutation of Species, via Wikimedia Commons.